1. Field of the Invention
The present invention relates to an image sensor and an image-capturing apparatus.
2. Description of the Related Art
There has been proposed an image-capturing apparatus which performs focus detection of an imaging lens by the phase-difference detection method using a two-dimensional image sensor including pixels each having a microlens formed in it.
Japanese patent Laid-Open No. 2000-156823 discloses an image-capturing apparatus including a plurality of pairs of focusing pixels arranged on a two-dimensional image sensor including a plurality of pixels. One pair of focusing pixels are configured to receive light fluxes from different areas in the exit pupil of the imaging lens using a light-shielding layer including an opening, thereby performing pupil division. Imaging signals are obtained by imaging pixels arranged in most parts of the two-dimensional image sensor, and a defocus amount is obtained based on signals from focusing pixels arranged in some parts of this image sensor, thereby allowing focus detection.
Also, Japanese Patent Laid-Open No. 2004-320270 discloses a CCD (image sensor) including low- and high-sensitivity photodiodes. In Japanese Patent Laid-Open No. 2004-320270, the capacitance of a floating diffusion (charge-accumulating layer) of a source follower circuit (amplifier circuit) is varied between the low- and high-sensitivity photodiodes, thereby reducing the difference in signal strength.
The focusing pixels described in Japanese Patent Laid-Open No. 2000-156823 partially shield light fluxes having passed through the exit pupil of the imaging lens using the light-shielding layer formed in the image sensor, thereby performing pupil division. Because the light-shielding layer does not influence any imaging pixel, the imaging pixel has a transmittance and light-reception efficiency higher than the focusing pixels. Therefore, a difference in saturation capacitance is generated between the imaging pixel and the focusing pixel.
To cope with this difference, the capacitances of floating diffusions of source follower circuits formed in the imaging pixel and the focusing pixel, respectively, can be made different.
However, the position of center of gravity of an area (pupil area) exhibiting a given pupil intensity distribution varies between the focusing pixel and the imaging pixel. Hence, the light-reception efficiency ratio between the focusing pixel and the imaging pixel is not constant, and varies considerably depending on, for example, the exit pupil distance (the distance from the exit pupil to the imaging surface) and a change in setting (aperture value) of an imaging optical system. This means that the difference in saturation capacitance between the imaging pixel and the focusing pixel cannot be sufficiently reduced simply by varying the capacitance of a floating diffusion in each individual type of pixel.